A conventional electroluminescence panel comprises a transparent substrate such as a glass plate, plural transparent electrodes such as indium tin oxide (ITO) provided on the transparent substrate, a first insulating layer such as a metal oxide or metal nitrides formed on the transparent electrodes, an emitting layer such as zinc sulfide doped with Mn or a rare earth element or an alkaline earth metal sulfide doped with selenium sulfide and a rare earth element provided on the first insulating layer, a second insulating layer of the same material as the first insulating layer formed on the emitting layer, and a plural back electrodes such as aluminum provided on the second insulating layer. The electroluminescence panel is used for a display device to display letters or figures by applying a determined voltage across selected transparent and back electrodes according to information supplied to the display device. Luminance and the ability to withstand applied voltages without breaking down are important properties of the panel. For this purpose, various electroluminescence panels have been proposed, especially, with regard to the characteristics of the insulating layers. In particular an electroluminescence panel preferably has the following properties
(1) The insulating layer has a high dielectric constance and breakdown voltage.
(2)The insulation layer is self-healing after breakdown.
(3) It is easy to inject charges into the emitting layer to increase the luminance.
(4) It is difficult to inject the charges into the insulation layers from electrodes.
Although it is difficult for a single insulating material to meet all of these requirements, Japanese Patent Kokai (laid-open) Nos. 58-29880 and 62-278794 have proposed electroluminescence panels which provide improvements in one or more of the above properties.
Japanese Patent Kokai No. 58-29880 discloses an electroluminescence panel which includes an insulating layer that comprises a material having a high dielectric constant and a perovskite structure. An additional layer of Y.sub.2 O.sub.3 having a thickness of 100 to 1000 .ANG. is provided between the insulating layer and an emitting layer. This facilitates injecting charges into the emitting layer and enhances luminance. Japanese Patent Kokai No. 62-278794 discloses three types of electroluminescence panels. Each type includes two insulating layers comprising a first insulating layer in contact with an emitting layer and a second insulating layer in contact with an electrode. In the first type of electroluminescence panel, the first insulating layer and the second insulating layer are formed from Ta.sub.2 O.sub.5 of low and high resistances, by respectively sputtering method. In the second type of electroluminescence panel, the second insulating layer comprises of SiO.sub.2 which is provided by an electron beam evaporation method. In the third type of electroluminescence panel, a first insulating layer of low resistivity Ta.sub.2 O.sub.5 is formed by an electron beam evaporation method.
However, the above described electroluminescence panels have certain disadvantages.
In the electroluminescence panel of Japanese Patent Kokai No. 58-29880, since a three element material having a perovskite structure is used for the insulating layer, it is difficult to maintain stoichiometry or obtain a predetermined dielectric constant, thereby reducing the breakdown voltage. Further, the breakdown mode is liable to propagate in the insulating layer.
In the first type of electroluminescence panel of Japanese Patent Kokai No. 62-278794, charges are easily injected into the second insulation layer of high resistivity Ta.sub.2 O.sub.5. Consequently, the ability of the panel to withstand applied voltages without breaking down is limited. Further, in manufacturing this panel, the ITO electrode tends to be black where the second insulating layer of Ta.sub.2 O.sub.5 is formed directly on the electrode by the sputtering method. In addition, the emitting layer is subject to damage and exfoliation of the film occurs where Ta.sub.2 O.sub.5 is formed by the sputtering method on zinc sulfide (ZnS), which is a mother material for the emitting layer.
In the second type of electroluminescence panel of Japanese Patent Kokai No. 62-278794, the electron beam evaporation method and the sputtering method are carried out one after the other in the fabrication process. As a result, the process is complicated. Further, the ability of the panel to withstand the applied voltages is limited, and the panel is likely to be contaminated due to the supply to and the discharge from the vacuum chamber during fabrication. In addition, this type of panel is subject to the same emitting layer damage and exfoliation as the first type.
In the third type of electroluminescence panel of Japanese Patent Kokai No. 62-278794, deviation of the composition tends to occur in the first insulating layer of Ta.sub.2 O.sub.5 based on the stoichiometry. Although the layer is suitable for a charge injection layer, the layer may need to be as thick as 10,000 .ANG. to provide as adequate breakdown voltage. In this case, the emitting threshold voltage and a driving voltage must unduly increased.